meta: Share directories with virtual machines

This *works*, and it's possible to edit files in one virtual machine
while having those files instantly be updated in all other virtual
machines. Note that the host will also have access to the files, which
ultimately means that directory sharing is quite useful (and convenient).

modules/virtualization.nix

@@ -2,6 +2,13 @@
   virtualisation = {
     memorySize = 8192;
     cores = 4;
+
+    sharedDirectories = {
+      testing = {
+        source = "/home/user/containers/testing";
+        target = "/mnt";
+      };
+    };
   };
 
   virtualisation.qemu.options = [